Sleep apnea is a physiological condition affecting millions of people worldwide. It is described as an iterated failure to respire properly during sleep. Those affected by sleep apnea stop breathing during sleep numerous times during the night. There are two types of sleep apnea, generally described in medical literature as central sleep apnea and obstructive sleep apnea. Central sleep apnea is a failure of the nervous system to produce proper signals for excitation of the muscles involved with respiration. Obstructive sleep apnea (OSA) is cause by physical obstruction of the upper airway channel (UAW).
Obstruction of the upper airway is associated with a depression of the respiratory system caused by a loss of tone of the oropharyngeal muscles involved in maintaining UAW patency. As those muscles lose tone, the tongue and soft tissue of the upper airway collapse, blocking the upper airway channel. Blockage of the upper airway prevents air from flowing into the lungs. This creates a decrease in blood oxygen level, which in turn increases blood pressure and heart dilation. This causes a reflexive forced opening of the UAW until the patient regains normal patency, followed by normal respiration until the next apneic event. These reflexes briefly arouse the patient from sleep (microarousals).
Current treatment options range from non-invasive approaches such as continuous positive applied pressure (CPAP) to more invasive surgical procedures such as uvulopalatopharyngoplasty (UPPP) and tracheostomy. In both cases patient acceptance and therapy compliance is well below desired levels, rendering the current solutions ineffective as a long term solution-for therapeutic treatment of OSA.
Implants are a promising alternative to these forms of treatment. Pharyngeal dilation via hypoglossal nerve (XII) stimulation has been shown to be an effective treatment method for OSA. The nerves are stimulated using an implanted electrode. In particular, the medial XII nerve branch (i.e., in genioglossus), has demonstrated significant reductions in UAW airflow resistance (i.e., increased pharyngeal caliber).
Reduced UAW airflow resistance, however, does not address the issue of UAW compliance (i.e., decreased UAW stiffness), another critical factor involved with maintaining patency. To this end, co-activation of both the lateral XII nerve branches (which innervate the hypoglossus (HG) and styloglossus (SG) muscles) and the medial nerve branch has shown that the added effects of the HG (tongue retraction and depression) and the SG (retraction and elevation of lateral aspect of tongue) result in an increased maximum rate of airflow and mechanical stability of the UAW.
While coarse (non-selective) stimulation has shown improvement to the AHI (Apnea+Hypopnea Index) the therapeutic effects of coarse stimulation are inconclusive. Selective stimulation of the functional branches is more effective, since each branch-controlled muscle affects different functions and locations of the upper airway. For example, activation of the GH muscle moves the hyoid bone in the anterosuperior direction (towards the tip of the chin). This causes dilation of the pharynx, but at a point along the upper airway that is more caudal (below) to the base of the tongue. In contrast, activation of the HG dilates the oropharynx (the most commonly identified point of collapse, where the tongue and soft palate meet) by causing tongue protrusion. Finally, the tongue retractor muscles (HG and SG) do not themselves generate therapeutic effects, but they have been shown to improve upper airway stability when co-activated with the HG muscle.
While electrical stimulation of the hypoglossal nerve (HGN) has been experimentally shown to remove obstructions in the UAW, current implementation methods require accurate detection of an obstruction, selective stimulation of the correct tongue muscles, and a coupling of the detection and stimulation components. Additionally, attempts at selective stimulation have to date required multiple implants with multiple power sources, and the scope of therapeutic efficacy has been limited. A need therefore exists for an apparatus and method for programmable and/or selective neural stimulation of multiple implants or contact excitation combinations using a single controller power source.